Glossary
Exit code
The number a command returns: 0 means success.
Every process ends by reporting a status number to its parent. Zero means success; anything else signals a failure, with specific values meaning specific things per program. The shell exposes the last one as $?.
This tiny convention is how independent programs coordinate: && runs the next command only on success, || only on failure, and if statements in scripts branch on it. Well-behaved scripts exit 1 on error so that they compose too.
See it in practice
$ grep -q root /etc/passwd && echo found || echo missinggrep prints nothing (-q) and communicates purely through its exit code, which && and || branch on.
Where you'll meet it
In-depth guide
SignalAn asynchronous message sent to a process, like SIGTERM or SIGKILL.
diffSee exactly what changed between two files or trees — the language of patches.